Acoustic structures

ABSTRACT

An acoustic structure includes a first, rigid panel ( 7 ), a second, rigid panel ( 11 ) aligned in spaced, substantially parallel, relationship with the first panel ( 7 ), a multiplicity of partition walls ( 15 ) running transverse to the panels of the panels ( 7, 11 ) and dividing the interior space of the enclosures into a single layer of cells ( 17 ) bounded at one face by the inside of the first panel ( 7 ) and bounded at the opposite face by the inside of the second panel ( 11 ), the partition walls ( 15 ) being bonded at the one face to the inside of the first panel ( 7 ) and at the opposite face to the inside of the second panel ( 11 ), and a multiplicity of apertures ( 19 ) in the partition walls ( 15 ) providing communication between adjacent cells of the single layer of cells ( 17 ).

[0001] This invention relates to acoustic structures.

[0002] In recent years, so-called flat panel loudspeaker units (the term“loudspeaker unit” being used to mean the combination of at least oneloudspeaker drive unit and a loudspeaker enclosure) have been introducedof which the overall depth is much reduced in comparison with aloudspeaker unit of traditional design. The reduced depth is possiblebecause mid-range and bass loudspeaker drive units with a reduced frontto back dimension have been developed.

[0003] It is, however, unfortunately true that savings of space inloudspeaker units and other acoustic apparatus for hi fi use generallyinvolve a reduction in the quality of the sound produced by theapparatus.

[0004] It is an object of the invention to provide an acoustic structurewhich can provide improved sound quality in acoustic apparatus ofrelatively small physical size.

[0005] The present invention provides an acoustic structure comprising:

[0006] a first rigid panel,

[0007] a second rigid panel aligned in spaced, substantially parallel,relationship with the first panel,

[0008] a multiplicity of partition walls running transverse to theplanes of the panels and dividing the interior space of the enclosureinto a single layer of cells bounded at one face by the inside of thefirst panel and bounded at the opposite face by the inside of the secondpanel, the partition walls being bonded at the one face to the inside ofthe first panel and at the opposite face to the inside of the secondpanel, and

[0009] a multiplicity of apertures in the partition walls providingcommunication between adjacent cells of the single layer of cells, andoptionally in which:

[0010] the cells each have a cross-sectional area parallel to the panelsin the range 0.25 to 10 cm²,

[0011] the apertures each have a cross-sectional area of at least 0.04cm², and

[0012] at least 55% of the wall between a cell and an adjoining cell isimperforate.

[0013] Such a structure especially with the dimensions given is capableof widespread usefulness in making hi fi acoustic apparatus such asloudspeaker enclosures, horn-type loudspeaker units, and labyrinth-typeloudspeaker units. The structure with the dimensions defined above has alow-pass filter characteristic analogous to the lumped capacitance andinductance equivalent circuit of an electrical transmission line, theapertures in the partition walls act as small masses (analogous toinductors) and the cells act as small springs (analogous to capacitors).The overall effect is that sound is delayed in passing from cell to cellvia the apertures and as result the acoustic structure can be providedas part of a horn, as part of a labyrinthine tube, and so on, to makethe acoustic apparatus produce sound giving the impression to the earthat the acoustic apparatus is physically larger than in fact itactually is.

[0014] An acoustic structure according to the invention can also be usedin making a loudspeaker enclosure, in which application it is highlyadvantageous because the resultant structure is very rigid although thefilter properties of the structure may not necessarily be made use of inthat application.

[0015] Thus, by means of an acoustic structure of the invention bettersound reproduction can be achieved for a given size of acousticapparatus. Fundamental tunings, such as the mass of a speaker conebouncing on the bulk stiffness of the enclosed air of a loudspeakerenclosure, or the mass of the main tuning port or auxiliary bassradiator also bouncing on the stiffness of enclosed air can remainessentially unchanged but other system resonances dependent on transittimes are affected beneficially.

[0016] An acoustic structure according to the invention may be producedas a product in its own right for insertion into acoustic apparatus, forexample, into a loudspeaker enclosure or it may be produced during themaking of acoustic apparatus so that the acoustic structure comes intobeing during the making of the acoustic apparatus. As an example of theformer case, a block of the acoustic structure may be made, cut to shapeand bonded to the interior of an acoustic apparatus. As an example ofthe latter case, a flat horn for a horn-type loudspeaker may be made bybonding a single layer of cells between two flat panels shaped to flarelike a horn. Thus, the horn and the acoustic structure are produced atone and the same time.

[0017] Preferably, the panels are flat panels but, in principle, theymay be of virtually any shape, for example, a curved or corrugatedshape.

[0018] Preferably, the spacing of the first and second panels is in therange 10 to 50 millimetres, more preferably in the range 15 to 35millimetres, and yet more preferably in the range 20 to 30 millimetres.

[0019] Preferably, the cells each have a cross-sectional area in therange 0.5 to 4 cm², more preferably a cross-sectional area in the range0.6 to 2 cm², and yet more preferably a cross-sectional area in therange 0.8 to 1.5 cm².

[0020] Preferably, the apertures each have a cross-sectional area of atleast 0.1 cm², more preferably in the range 0.15 to 0.25 cm².

[0021] Preferably, at least 60% of the wall between a cell and anadjoining cell is imperforate, more preferably, at least 70% of the wallbetween a cell and an adjoining cell is imperforate.

[0022] The diameter of the cells is preferably between 15 and 50millimetres, more preferably between 20 and 30 millimetres. Such cellsizes give good acoustic results in a mid-range or bass loudspeakersystem.

[0023] The above dimensions give good practical results at thefrequencies used in hi fi apparatus.

[0024] Sound absorbent material may be provided within some or all ofthe cells.

[0025] The apertures may be in the form of slots at the edges of thepartition walls. The slots may be at some or all of the edges of thepartition walls but instead holes of virtually any shape may be providedvirtually anywhere on the partition walls.

[0026] Advantageously, each cell has two walls parallel to each otherdefined by parts of the metal partition walls, and apertures areprovided in the said two walls parallel to each other.

[0027] Preferably, the apertures are arranged in pairs, one aperture ofeach pair being adjacent the first panel and the other being adjacentthe second panel.

[0028] Advantageously, the arrangement of the apertures is non-uniform.For example, apertures can be provided to a greater degree along apreferred axis of sound travel such as the long dimension of anenclosure or horn.

[0029] Preferably, the partition walls are formed by a multiplicity ofinter-connected lamellae expanded into a network of cells. That featuremakes manufacture particularly simple.

[0030] It is preferred that the panels are made of a material having aYoung's modulus greater than 50 GPa. A high Young's modulus isparticular advantageous when making a loudspeaker enclosure in order toobtain high rigidity.

[0031] It is also preferred that the partition walls are made of amaterial having a Young's modulus greater that 50 GPa.

[0032] Advantageously, the panels are made of glass. Glass is a materialcapable both of contributing great rigidity to the structure and ofproviding an aesthetically attractive finish. Clear glass may be used togive an interesting view into the interior of the structure. The glassmay be, for example, between 2 and 10 millimetres thick, more preferablybetween 4 and 8 millimetres thick, and yet more preferably approximately6 millimetres thick. Toughened glass may be used to increase physicalsafety. The glass may be laminated to provide both acoustic damping andphysical safety.

[0033] The panels may instead be made of metal.

[0034] The panels and/or the partition walls may be made of aluminium.In that way, stiffness and lightness can be combined.

[0035] The metal panels may be between half a millimetre and twomillimetres thick. That combines sufficiency of stiffness with economyof metal, and lightness and also avoids loss of internal volume.

[0036] Preferably, the partition walls are less than one tenth thethickness of the panels. By that means, good rigidity can be combinedwith economical use of material and lightness.

[0037] The cells are preferably polygonal. They may be hexagonal, basedon either regular or elongated hexagons. Polygonal cells are easy tomanufacture and hexagonal cells give particular rigidity.

[0038] Advantageously, the partition walls are adhesively bonded to thepanels, preferably by means of an adhesive having low resilience whenset, for example, an epoxy resin adhesive. That is a particularly simplemanufacturing technique and the choice of a low resilience adhesive hasan advantageous effect on sound quality although some resilience in theadhesive may be used for acoustic damping.

[0039] Advantageously, at least for some applications, at least three ofsaid flat panels in spaced, substantially parallel relationship areprovided, there being a respective single layer of cells between eachadjacent pair of panels, the or each panel that lies between twoadjacent layers of cells including a multiplicity of apertures providingcommunication between cells of the adjacent layers of cells. By thatmeans, sounds can be delayed when passing from front to back, from sideto side, and up and down in the structure. Labyrinthine, meandering andother sound routes can be defined by suitable placing of communicationapertures.

[0040] The number of flat panels may be selected from the groupconsisting of 2, 3, 4, 4 or more, and 5 or more, flat panels.

[0041] Advantageously, an enclosure for a loudspeaker drive unitcomprises a structure as claimed in any preceding claim, wherein one ofthe panels forms the front of the enclosure and has an opening thereinfor mounting a loudspeaker drive unit, a peripheral wall running aboutthe periphery of the structure to enclose the space within thestructure. It is of very great advantage from the point of view of soundreproduction that such a cellular enclosure construction is very stiff.

[0042] Preferably, the peripheral wall is made of metal.

[0043] The peripheral wall may be made of aluminium but it could insteadbe made of a plastics material. For example, plastics material mouldedinto a C-shaped cross-section and filled with foamed plastics or othermaterial may be used.

[0044] The panels may be rectangular panels but many other shapes arepossible.

[0045] The overall depth of the enclosure may be less than 50millimetres, less than 40 millimetres, less than 30 millimetres, or lessthan 20 millimetres, or between 10 and 15 millimetres.

[0046] Each panel may have an overall area of between 500 and 4,000square centimetres, or between 1,000 and 3,000 square centimetres.

[0047] Acoustic apparatus including an acoustic structure in accordancewith the invention will now be described, by way of example only, withreference to the accompanying drawing, in which:

[0048]FIG. 1 is a diagrammatic cross-section through a flat panelloudspeaker unit;

[0049]FIG. 2 shows a network of cells used in the enclosure of theloudspeaker unit.

[0050]FIG. 3 is a diagrammatic illustration of a single cell identifyingits dimensions;

[0051]FIG. 4 is a front view of a second flat panel loudspeaker unit;

[0052]FIG. 5 is a diagrammatic perspective view of a first horn-typeloudspeaker unit including an acoustic structure in accordance with theinvention for providing a filter characteristic;

[0053]FIG. 6 is a diagrammatic side view corresponding to FIG. 5;

[0054]FIG. 7 is a diagrammatic cut-away end view corresponding to FIG.5;

[0055]FIG. 8 is a diagrammatic cross-sectional plan view correspondingto FIG. 5;

[0056]FIG. 9 is a diagrammatic plan view of a second horn-typeloudspeaker unit including an acoustic structure in accordance with theinvention for providing a filter characteristic;

[0057]FIG. 10 is a diagrammatic end view corresponding to FIG. 9; and

[0058]FIG. 11 is a diagrammatic perspective view of an acoustic hornincluding an acoustic structure in accordance with the invention forproviding a filter characteristic.

[0059] Referring to the accompanying drawing, a loudspeaker unit 1comprises a loudspeaker drive unit 3 of the modern reduced physicaldepth type mounted in an enclosure 5. The loudspeaker drive unit 3 canbe either a mid-range or a bass unit. The enclosure 5 comprises a first,flat, metal panel 7 forming the front of the enclosure and having anopening 9 therein in which the loudspeaker drive unit 3 is mounted. Theenclosure 5 further comprises a second, flat, metal panel 11 aligned inspaced, substantially parallel, relationship with the first metal panel7 and forming the rear of the enclosure. A peripheral wall 13 runs aboutthe periphery of the first and second metal panels 7, 11 to enclose thespace therebetween, the peripheral wall running transverse to the planesof the metal panels and being bonded at the front to the first metalpanel and at the rear to the second metal panel. Epoxy resin is asuitable adhesive for securing the peripheral wall 13 in place.

[0060] A multiplicity of metal partition walls 15 run transverse to theplanes of the metal panels 7, 11 and divide the interior space of theenclosure into a single layer of cells 17 bounded at the front by theinside of the first metal panel 7 and bounded at the rear by the insideof the second metal panel 11, the partition walls being bonded at thefront to the inside of the first metal panel and at the rear to theinside of the second metal panel.

[0061] A multiplicity of apertures 19 (not shown in FIG. 1) in the metalpartition walls 15 provide communication between adjacent cells of thesingle layer of cells 17.

[0062] The partition walls 15 are formed by a multiplicity ofinter-connected lamellae expanded into a network of cells as shownschematically in FIG. 2. The expansion of the lamellae into a network ofcells is analogous to the way in which paper Christmas directions can beopened up from a compressed state.

[0063] Both the panels 7 and 11 and the partition walls 15 are made ofaluminium, the metal panels being approximately one millimetre thick andthe partition walls being a little less than 0.1 millimetre inthickness.

[0064] As can be seen in FIG. 2, the cells are hexagonal, the hexagonsbeing regular hexagons.

[0065] When constructing the enclosure 5, the partition walls 15 areadhesively bonded to the panels by means of an epoxy resin adhesive.

[0066] The peripheral wall 13 is also made of metal, namely, aluminium.It is in the form of a strip of metal of length corresponding to theperiphery of the panels, bent to shape and bonded into place.

[0067] The panels 7 and 11 are rectangular panels and the overall depthof the enclosure is approximately 25 millimetres so that the system is aso-called “flat panel” system. The diameter of the cells (side toopposite side measurement) is approximately 25 millimetres.

[0068] If desired, sound absorbent material (not shown) can be providedwithin some or all of the cells of the layer of cells 17.

[0069] The apertures 19 are in the form of slots at the edges of thepartition walls as shown in FIG. 2. The apertures can be provided insome or all sides of the cells so as to communicate in some or alldirections with adjacent cells. As seen in FIG. 2, each cell has twowalls 21 parallel to each other in which the apertures 19 are provided.As seen in FIG. 2, the apertures are arranged in pairs 23A, 23B, oneaperture of each pair being at the front and the other being at the rearof the metal partition walls 15. Many other arrangements of aperturesare, however, possible such as apertures in the central regions of thecell walls. Holes with dimensions which change with distance from aloudspeaker drive unit according to some desire law, for example, alogarithmic law can be provided.

[0070] The overall dimensions of the enclosure 5 are 650×300×25millimetres approximately and thus each metal panel has an overall areaof approximately 1,950 square centimetres.

[0071] The construction shown has the advantage that the distance fromthe speaker diaphragm to the rear of the enclosure is relatively shortso that standing waves in that direction within the cells are not aproblem (as they can be in known speakers of which the interior isdivided into cells).

[0072] Instead of making the partition walls 13 separately from thepanels 7 and 11, it is possible to form them integrally with one of thepanels by die-casting and then to secure the remaining panel by adhesivebonding. In that case, the partition walls are integrally bonded to onepanel and adhesively bonded to the other. The partition walls are notnecessarily arranged normal to the panels but may be at an angle tothem. For example, a single three-dimensional sheet of material havingpeaks and pits in the manner of a conventional egg tray can be used tocreate sloping partition walls. The pits which in an conventional eggtray would hold the eggs form the cells and the spaces between the peaksform the apertures between cells. Apertures could be provided connectingone side of the single sheet to the other.

[0073] If desired, one or more reflex ports or one or more ABRs(auxiliary bass radiators) can be included in one of the panels. TheABRs may be of conventional form or as described in our specification WO00/32010.

[0074] The peripheral wall can, if desired, be formed by the outermostpart of the partition walls rather than being a separate component inits own right.

[0075] The acoustic effects of the structure depend upon thedimensioning of the cells and apertures. FIG. 3 shows a single cell 20with especially advantageous dimensions for creating the delay effectintended when the cells of the structure conform to these dimensions.FIG. 3 also shows that the placing of apertures 22 does not necessarilyhave to be uniform. The marked dimensions identified by letters are asfollows: dimension millimetres a 25 b 6 c 10 d 5 e 4

[0076]FIG. 4 shows a flat panel type loudspeaker unit 30 comprising tworectangular panels of transparent glass 32 (only one is visible in thedrawing), a single layer of hexagonal cells 34 sandwiched between thepanels and bonded to them, a peripheral wall 36 about the cells andbonded to the panels, a tweeter drive unit 38, a mid-range drive unit40, and two bass drive units 42. The units 40 and 42 are again of themodern reduced depth type (the tweeter drive unit 38 being of ordinaryconstruction and having relatively little depth). The loudspeaker unit30 in general construction corresponds to what has already beendescribed with reference to FIGS. 1, 2 and 3 but includes moreloudspeaker drive units and is of see-through construction. If desired,one or more of the loudspeaker units could be replaced by an ABR.

[0077]FIG. 5 shows a horn-type speaker 50 comprising a loudspeaker driveunit 52, a throat portion 54, and a flat horn 56 comprising an acousticstructure according to the invention providing a filter characteristic.The flat horn comprises two plates 58 and 60 sandwiching a single layerof apertured cells 62 as in the constructions of FIGS. 1 to 4 but inthis instance the plates are shaped to flare like a horn and theloudspeaker drive unit is applied at an edge of the acoustic structure.The sides of the horn are closed by side walls 64 and 66 curved tofollow the flare of the horn.

[0078]FIG. 6 gives a diagrammatic representation of the side of the hornwith the associated side wall 66 cut away to reveal the internalstructure.

[0079]FIG. 8 indicates that the apertures in the cells are arranged topermit sound transmission, as indicated by the arrow 70, along the axisof the horn, and crosswise to the axis as indicated by the double-headedarrow 66.

[0080] The use of an acoustic structure in accordance with the inventionfor providing a filter characteristic in a horn-type speaker provides aneffective gain in the length of horn for a given output. It appears thatthe acoustic structure lowers the effective “m” or flare rate of thehorn and hence the low frequency “cut-off” frequency is lowered for agiven physical length and given flare rate.

[0081]FIGS. 9 and 10 show another horn-type speaker comprising twosquare panels 82 and 84 sandwiching a single layer of apertured cells 86as in the earlier figures providing a filter characteristic. In thisinstance, however, a loudspeaker drive unit 88 is mounted on one outerface to send sound into the interior of sandwich structure to emerge atthe four sides of the square as if from four horns. The effect of fourhorns is achieved by internal walls 90 closing the sides of each horn ina generally similar manner to the side walls 64 and 66 in theconstruction of FIGS. 5, 6 and 7. If desired, the spaces 92 between thefour horns can be acoustically coupled to the rear of the diaphragm ofthe loudspeaker driver 88 to provide a four-lobed rear chamber.

[0082] The structures described so far with reference to the drawingshave all consisted of a single layer of cells between two outer panels.It is also possible to make an acoustic structure in accordance with theinvention in the form of a “multi-layer sandwich”, that is to say, astructure in which a multiplicity of panels are provided with a singlelayer of apertured cells between each adjacent pair of panels. Internalpanels are provided with apertures, sized similarly to the apertures inthe cells, to provide sound communication from layer to layer of thesandwich. A “block” of acoustic structure can be made in this way forfitting into an acoustic structure of known form. The loudspeaker unitsshown in FIGS. 1 to 10 could also be made in multi-layer instead ofsingle layer form.

[0083]FIG. 11 shows an acoustic horn 100 filled with a block 102 ofmulti-layer acoustic structure in accordance with the invention forproviding a filter characteristic. The block is cut to shape and bondedinto place in the horn. The acoustic horn 100 can be used, for example,as the front horn of a horn-type loudspeaker unit or as a rear tube in aspeaker of the type described in GB 2 290 672A or in WO 98/51121.

[0084] In FIG. 11, the cells are shown out of alignment from one layerto the next as might arise in practical construction but one layer couldequally well align with the next.

[0085] Many different materials can be used in making an acousticstructure in accordance with the invention.

[0086] For example, paper card, Aramid paper with phenolic coating,epoxy woven glass fabric, aluminium alloy, and epoxy woven carbon fabriccan be used for making the cells. The papers and fabrics can, forexample, be used in thicknesses of 0.05 to 0.5 millimetres and aluminiumcan, for example, be used in thicknesses of 0.025 to 0.15 millimetres.

[0087] For example, tempered hardwood veneer, aluminium alloy, carbonfibre epoxy resin composite panel, glass and steel can be used formaking the panels with thicknesses, for example, of 0.5 to 13millimetres.

[0088] An acoustic structure of the invention can be incorporated intovirtually any acoustic apparatus of suitable size.

1. An acoustic structure comprising: a first, rigid panel, a second,rigid panel aligned in spaced, substantially parallel, relationship withthe first panel, a multiplicity of partition walls running transverse tothe planes of the panels and dividing the interior space of theenclosure into a single layer of cells bounded at one face by the insideof the first panel and bounded at the opposite face by the inside of thesecond panel, the partition walls being bonded at the one face to theinside of the first panel and at the opposite face to the inside of thesecond panel, and a multiplicity of apertures in the partition wallsproviding communication between adjacent cells of the single layer ofcells, and, optionally, in which: the cells each have a cross-sectionalarea parallel to the panels in the range 0.25 to 10 cm², the apertureseach have a cross-sectional area of at least 0.04 cm², and at least 55%of the wall between a cell and an adjoining cell is imperforate.
 2. Astructure as claimed in claim 1, wherein the spacing of the first andsecond panels is in the range 10 to 50 millimetres.
 3. A structure asclaimed in claim 2, wherein the spacing of the first and second panelsis in the range 15 to 35 millimetres.
 4. A structure as claimed in claim3, wherein the spacing of the first and second panels is in the range 20to 30 millimetres.
 5. A structure as claimed in any preceding claim,wherein the cells each have a cross-sectional area in the range 0.5 to 4cm².
 6. A structure as claimed in claim 5, wherein the cells each have across-sectional area in the range 0.6 to 2 cm².
 7. A structure asclaimed in claim 6, wherein the cells each have a cross-sectional areain the range 0.8 to 1.5 cm².
 8. A structure as claimed in any precedingclaim, wherein the apertures each have a cross-sectional area of atleast 0.1 cm^(2 .)
 9. A structure as claimed in claim 8, wherein theapertures each have a cross-sectional area in the range 0.15 to 0.25cm².
 10. A structure as claimed in any preceding claim, wherein at least60% of the wall between a cell and an adjoining cell is imperforate. 11.A structure as claimed in claim 10, wherein at least 70% of the wallbetween a cell and an adjoining cell is imperforate.
 12. A structure asclaimed in any preceding claim, wherein the diameter of the cells isbetween 15 and 50 millimetres.
 13. A structure as claimed in claim 12,wherein the diameter of the cells is between 20 and 30 millimetres. 14.A structure as claimed in any preceding claim, wherein sound absorbentmaterial is provided within some or all of the cells.
 15. A structure asclaimed in any preceding claim, wherein the apertures are in the form ofslots at the edges of the partition walls.
 16. A structure as claimed inany preceding claim, wherein each cell has two walls parallel to eachother defined by parts of the partition walls, and apertures areprovided in the said two walls parallel to each other.
 17. A structureas claimed in any preceding claim, wherein the apertures are arranged inpairs, one aperture of each pair being adjacent the first panel and theother being adjacent the second panel.
 18. A structure as claimed in anypreceding claim, wherein the arrangement of the apertures isnon-uniform.
 19. An enclosure as claimed in any preceding claim, whereinthe partition walls are formed by a multiplicity of inter-connectedlamellae expanded into a network of cells.
 20. A structure as claimed inan preceding claim, wherein the panels are made of a material having aYoung's modulus greater than 50 GPa.
 21. A structure as claimed in anypreceding claim, wherein the partition walls are made of a materialhaving a Young's modulus greater that 50 GPa.
 22. A structure as claimedin of claims 1 to 19, wherein the panels are made of glass.
 23. Astructure as claimed in any of claims 1 to 19, wherein the panels aremade of metal.
 24. A structure as claimed in claim 23, wherein thepanels and/or the partition walls are made of aluminium.
 25. A structureas claimed in claim 23 or claim 24, wherein the panels are between halfa millimetre and two millimetres thick.
 26. A structure as claimed inany preceding claim, wherein the partition walls are less than one tenththe thickness of the panels.
 27. A structure as claimed in any precedingclaim, wherein the cells are polygonal.
 28. A structure as claimed inclaim 27, wherein the cells are hexagonal.
 29. A structure as claimed inany preceding claim, wherein the partition walls are adhesively bondedto the panels.
 30. A structure as claimed in claim 29, wherein thepartition walls are bonded by means of an adhesive having low resiliencewhen set.
 31. A structure as claimed in claim 30, wherein the partitionwalls are bonded by means of an epoxy resin adhesive.
 32. A structure asclaimed in any preceding claim, wherein at least three of said flatpanels in spaced, substantially parallel relationship are provided,there being a respective single layer of cells between each adjacentpair of panels, the or each panel that lies between two adjacent layersof cells including a multiplicity of apertures providing communicationbetween cells of the adjacent layers of cells.
 33. A structure asclaimed in claim 32, wherein the number of flat panels is selected fromthe group consisting of 2, 3, 4, 4 or more, and 5 or more, flat panels.34. An enclosure for a loudspeaker drive unit comprising a structure asclaimed in any preceding claim, wherein one of the panels forms thefront of the enclosure and has an opening therein for mounting aloudspeaker drive unit, a peripheral wall running about the periphery ofthe structure to enclose the space within the structure.
 35. Anenclosure as claimed in claim 34, wherein the peripheral wall is made ofmetal.
 36. An enclosure as claimed in claim 35, wherein the peripheralwall is made of aluminium.
 37. An enclosure as claimed in any of claims34 to 36, wherein the panels are rectangular panels.
 38. An enclosure asclaimed in any of claims 34 to 37, wherein the overall depth of theenclosure is less than 40 millimetres.
 39. An enclosure as claimed inclaim 38, wherein the overall depth of the enclosure is less than 20millimetres.
 40. An enclosure as claimed in claim 39, wherein theoverall depth of the enclosure is between 10 and 15 millimetres.
 41. Anenclosure as claimed in any of claims 34 to 40, wherein each panel hasan overall area of between 500 and 4,000 square centimetres.
 42. Anenclosure as claimed in claim 41, wherein each metal panel has anoverall area of between 1,000 and 3,000 square centimetres.
 43. Ahorn-type loudspeaker unit, wherein the interior of the horn contains astructure as claimed in any of claims 1 to
 33. 44. A labyrinth-typeloudspeaker unit wherein labyrinthine passage or tube behind aloudspeaker drive unit contains a structure as claimed in any of claims1 to
 33. 45. A loudspeaker system comprising an enclosure as claimed inany of claim 34 to 42, and a loudspeaker drive unit mounted in the saidopening.
 46. A loudspeaker system as claimed in claim 45, wherein thesystem is a flat panel loudspeaker system.